Codex/libcamera rpi fix

[jeremymcgee73]

Summary

Fix Raspberry Pi camera userspace startup on OpenMANET, and make the camera streaming service default to the OpenMANET HaLow-facing interface on Pi builds.

This MR contains the package-side changes required by the corresponding firmware-side Raspberry Pi camera enablement.

What this changes

libcamera

• add a patch to guard the RPi IPA frame-length queue on first AGC application:
  • multimedia/libcamera/patches/0006-rpi-guard-empty-frame-length-queue.patch

This prevents an abort in the Raspberry Pi camera pipeline where applyAGC() could call frameLengths_.pop_front() before the queue had been populated.

bcm271x board defaults

• add a Pi-only first-boot default:
  • boards/bsp-bcm271x/files/uci-defaults/20_camera-onvif-server-interface
• add a Pi-only preserved-config migration:
  • boards/bsp-bcm271x/files/hotplug.d/iface/95-camera-onvif-server-interface

These changes make camera-onvif-server use ahwlan on OpenMANET Pi images instead of the generic lan default.

Why

Before these changes:

• the Pi camera could enumerate successfully but abort when a real stream started, due to an empty frame-length queue in the RPi IPA path
• camera-onvif-server defaulted to lan
• on OpenMANET Pi images, the active network is often ahwlan, so the service failed with:
  • interface lan not ready
• preserved-config upgrades could keep the stale lan setting even after the image layout moved to ahwlan

After these changes:

• the Pi camera can start a real libcamera streaming path without hitting the deque assertion
• fresh Pi flashes default the camera service to ahwlan
• preserved-config upgrades self-correct once ahwlan comes up

Scope

These interface-default and migration changes are intentionally limited to:

• bsp-bcm271x

They are not applied globally to:

• Venice
• HD01
• other non-Pi OpenMANET boards

Validation

Validated on Raspberry Pi 4 / bcm2711 with IMX219:

• cam -l reports the camera successfully
• MediaMTX creates the rpicamera path
• RTSP streaming works via:
  • rtsp://<device-ip>:554/rpicamera

Observed failure before the libcamera fix:

• process aborted in std::deque::pop_front() from the RPi IPA path during stream startup

Observed failure before the interface-default fix:

• camera-onvif-server attempted to use lan
• MediaMTX did not configure the rpicamera path
• RTSP clients received:
  • path 'rpicamera' is not configured

Observed success after these changes:

• camera service bound to ahwlan
• dynamic rpicamera path created
• RTSP path usable from clients

Notes

This MR is intended to pair with the corresponding firmware MR that:

• enables the bcm27xx kernel/image support for Raspberry Pi CSI cameras
• pins the openmanet feed to the package commit containing these fixes